General microbiology and sterilisation: porin loss: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · General microbiology and sterilisation: porin loss

Sample item

During therapy of Pseudomonas aeruginosa pneumonia, the isolate becomes resistant to imipenem but stays susceptible to ceftazidime and piperacillin-tazobactam. No carbapenemase is detected. Which mechanism is most likely?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Loss of the OprD outer membrane porin

Imipenem enters Pseudomonas through the OprD porin. Mutational loss of OprD lowers imipenem uptake and gives resistance while other beta-lactams that use different routes remain active.

Option by option

  • A. Acquisition of the altered PBP2a

    PBP2a is the methicillin-resistance protein of Staphylococcus aureus, not Pseudomonas.

  • B. Ribosomal methylation by an erm gene

    erm methylation causes macrolide-lincosamide-streptogramin resistance and not beta-lactam resistance.

  • C. Production of an extended-spectrum beta-lactamase

    An ESBL hydrolyses cephalosporins such as ceftazidime and spares carbapenems.

  • D. Loss of the OprD outer membrane porin (correct)

    Correct. Loss of OprD reduces imipenem entry and selectively raises its MIC.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Loss of the OprD outer membrane porin. Imipenem enters Pseudomonas through the OprD porin.

Why is "Acquisition of the altered PBP2a" not the answer?

PBP2a is the methicillin-resistance protein of Staphylococcus aureus, not Pseudomonas.

Why is "Ribosomal methylation by an erm gene" not the answer?

erm methylation causes macrolide-lincosamide-streptogramin resistance and not beta-lactam resistance.

Why is "Production of an extended-spectrum beta-lactamase" not the answer?

An ESBL hydrolyses cephalosporins such as ceftazidime and spares carbapenems.

What concept does this question test?

It tests General microbiology and sterilisation: porin loss in Microbiology. Mutational loss of OprD lowers imipenem uptake and gives resistance while other beta-lactams that use different routes remain active.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General microbiology and sterilisation: porin loss?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.